A few ways but the simplest (but not so accurate) is to use an inductor (the smaller ones that look like resisors), 1mh or be more than sufficient, in place of the coil meter.
Put your arduino's adc input to the transistor's collector. The voltage across the inductor is proportional to the current, and is an average if the inductor is of sufficient size.
BTW, the circuit can be greatly simplified by your arduino.